Portal effect in EEVEE?
View and Render Layers are working similar to 2.79 but sadly EEVEE can't use Material/Object Index Pass. So you can't make a portal using this method. Read more here and watch video from the links: Blender 2.8 - Object index pass in Eevee

How to replicate it using different method




I've created two Collections - Portal and Behind_portal and also two View Layers accordingly.


To exclude Collection from View Layer - select it and press E or RMB > View Layer > Set Exclude.
In Compositing I've used Color Key to "remove" color of Holdout shader assigned to portal. I've also scaled a bit result and blurred it, as Color Key is not 100% precise.